Perhaps the biggest metaphysical problem concerning existence is why anything should exist at all - why there should be something rather than nothing. Physicists can maybe tell us why matter exists, adverting to conditions obtaining shortly after the Big Bang; perhaps they can even explain the existence of time and space, if this is as intimately linked to the existence of matter and energy as modern cosmologists suggest. But the metaphysical question clearly goes beyond such merely empirical considerations. One response is to say that the question is absurd, because it erroneously presupposes that we can make sense of the idea of absolute nothingness as a genuine alternative to the existence of at least something. On this view, we mistake the contingency of the things that do exist for a contingency in the fact that anything whatever exists. However, while it may indeed be impossible to imagine a world in which nothing exists, the notion of a wholly empty world does not seem obviously incoherent.

 http://www.formalontology.it/existence.htm

What is existence?

http://www.rci.rutgers.edu/~owl/rand2.html

OBJECTIVISM AND THE PRIMACY OF EXISTENCE

http://existence.srichinmoyart.com/

Sri Chinmoy on existence

http://webcenter.ru/~korn/english/

Mechanisms of Consciousness and Existence

http://www.atheists.net/pages/simple.html

The Simplicity of Existence